The present invention relates to an apparatus used in the field of optical computing and optical measurement which performs an optical correlation processing using a coherent beam on a two-dimensional image obtained from an imaging device such as a CCD camera to automatically perform pattern recognition and measurement.
Conventionally, the optical pattern recognition apparatus and correlation processing apparatus generally employ a joint transform correlator. As shown, for example, in FIG. 9, a spatial light modulator of the light addressed type is utilized in such apparatus, as disclosed in Japanese Patent Application Laid-Open Nos. 138616/1982, 210316/1982 and 21716/1983. In the FIG. 9 apparatus, a plate 95 has a joint image composed of a reference image, i.e., recognition basis and an input image, i.e., an object of recognition. A laser 91 emits a laser beam which is expanded by a beam expander 92. Thereafter, the expanded laser beam irradiates the joint image on the plate 95 to convert the joint image into a coherent image. The thus obtained coherent image is Fourier transformed by a Fourier transform lens 96. Light intensity distribution of the Fourier transform image is recorded on a spatial light modulator in the form of a liquid crystal light valve 97 which is disposed on a Fourier plane of the lens 96.
Next, an incident light beam divided by a beam splitter 93 is directed through mirrors 114,115 and a polarizing beam splitter 98 onto the liquid crystal light valve 97 to read out the recorded light intensity distribution of the Fourier transform image. The thus reproduced Fourier transform image is passed through the polarizing beam splitter 98 and is again Fourier transformed by another Fourier transform lens 99 to produce on its Fourier transform plane a correlation image containing correlation peaks indicative of correlation coefficient between the input image and the reference image. Such peak is detected by a CCD camera 100.
FIG. 10 shows an example of a joint image composed of an input image and a reference image adjacent to the input image. FIG. 11 shows an example of a pair of correlation peaks detected by the CCD camera 100 and indicative of a correlation coefficient between the reference image and the input image.
However, the conventional apparatus normally treats a pair of a single input image and a single reference image. Therefore, when recognizing a letter of alphabet, a particular character to be recognized is represented as an input image. In order to examine correlation relative to all of the alphabetical characters, a reference image is replaced one by one for each character so as to effect sequential correlation processing, thereby consuming considerable time. In order to solve such problem, concurrent correlation processing could be undertaken between an input image and a multiple of reference images. However, in such case, intensity of each correlation peak is seriously weakened due to interference during concurrent correlation processing between the multiple of the reference images and a single input image, while noise increases to undermine the weakened correlation peaks and to hinder separation thereof to thereby cause incorrect recognition.
Further recently, it has been suggested, for example, in B. Javidi and C. J. Kuo, Applied Optics, 27,663(1988) to binarize the intensity distribution of the joint Fourier transform image between reference and input images to form clear and sharp correlation peaks to improve the S/N ratio. However, this prior art construction is still not effective to carry out the concurrent correlation processing without any incorrect recognition.